It's a personal-taste thing, and personally, I like to unbutton my cuff and roll it tightly until it just clears the crook of my elbow. It's partly for comfort and partly to keep my shirt from dirtying up on the detritus of my desk. I have a messy desk.

So long as you take care to get it right rather than looking as if you just spent the night in a local jail. First up, your collar needs to be more stiff and more formal, with the ability to stand up on its own without the support of a tie. (Softer, lower collars tend to collapse in on themselves when undone.) Similarly, your suit needs to fit without acres of extra cloth flapping about. The rest is all about the way you carry yourself.